We will close the gaps in our knowledge of the neural mechanisms for lordosis behavior, and estrogen and progesterone effects on it in female rats. We aim to provide a complete neural and endocrine explanation of this component of reproductive behavior. I. To characterize the estrogen effect on hypothalamic cells more precisely we will use a quantitative application of steroid hormone autoradiography to test ideas of the estradiol uptake in specific cells: Is it active (vs. random, Poisson)? Is it quantal (vs. continuous)? Combining autoradiography with other techniques will tell us more about specific estrogen-binding neurons. Where do they project (autoradiography combined with HRP)? What important substances do they contain (autoradiography combined with HRP)? What important substances do they contain (autoradiography combined with immunocytochemistry)? Microinjection experiments will reveal the nature of behaviorally important ventromedial hypothalamic (VMH) actions. Are they electrical, secretory, or both? Single unit recording in VMH will characterize progesterone effects on these cells. II. Hypothalamic output must be registered in the midbrain. Electrical stimulation and microinjection of cells in and adjacent to the midbrain central grey will test the electrical and possible (modulatory) secretory actions here for their importance for lordosis behavior. Possible (synergistic) interactions of secretory with electrical processes in central grey will also be tested. LHRH in midbrain central grey can facilitate lordosis. We will analyze the relation of this behavioral LHRH effect to (a) LHRH chemical structure, (b) time of day, and (c) steroid hormone levels. III. Tracts descending from brainstem to spinal cord important for lordosis are the medullary (lateral) reticulospinal tract (RST) and the lateral vestibulospinal tract (LVST). We will analyze the RST role by chemical destruction of medullary reticular neurons and by electrical stimulation. The "trading relations" of LVST electrical signals with hormone levels will be studied by systematically varying LVST stimulation current, estradiol dose and progesterone dose. Finally, normal electrical activity of single units during lordosis and other behaviors will be demonstrated by chronic recording from individual RST and (in separate experiments) LVST neurons.